Counterion condensation on polyelectrolyte chains adsorbed on charged surfaces

Abstract

We developed a Monte Carlo systematic study into the effect of counterion condensation on the polyelectrolyte adsorption process on charged surfaces. Polyelectrolyte is modeled as a full-flexible chain whose size is characterized by the equilibrium bond length and the number of monomers per chain. The small anions and cations are explicitly modeled. The adsorption proceeds with a non-trivial counterion condensation degree on the polyelectrolyte chain. When the polyelectrolyte coverage degree on the surface is low, the chain looses most of its counterions (anions), due to their electrostatic repulsion with the negatively charged surface. This effect is more evident when the equilibrium bond length is shorter. Counterions are recondensed as coverage degree increases, and this is attributed to two main energetic reasons: first, the chains adsorbed cause shielding of anion-surface repulsive electrostatic interaction; second, the repulsive interaction between chains adsorbed on the surface is shielded by the condensed counterions on these chains. The amount of polyelectrolyte adsorbed and degree of condensation counterions reaches a plateau as a function of the number of chains added to the simulation box. At this point surface charge is overcompensated in a similar magnitude for the different chain types. However, the adsorbed chains keep most of their condensed counterions when the equilibrium bond length is shorter. Additionally, we study the size effect (number of monomer per chain) on the condensation degree on adsorbed polyelectrolyte. Condensation is highly dependent on a low chain size (low than 60 monomers approximately) since end effects are important. With a higher chain size, counterion condensation and charge reversal show a negligible correlation with chain size.